Comparison of different metal sulfate catalysts
In this study, the role of a series of metal sulfate catalysts on the
oxidative lignin depolymerization was evaluated preliminarily. The
effect of H2SO4 on the oxidation of
lignin was considered in the present of CuSO4 (point c
in Figure 3), as it has reported that BrĂ²nsted acid is an effective
promoter for lignin oxidation.43 Experimental results
exhibit that the existence of H2SO4(12.5 mmol L-1) in the microemulsion system does
promote the oxidation of lignin and enhance the yield of phenol monomers
(61.9 to 89.3 mg g-1, Table 3, entries 1 and 4).
Therefore, the above microemulsion containing 12.5 mmol
L-1 H2SO4 was used
while other cheaper and water-soluble metal sulfates with divalent
cations (CuSO4, MgSO4,
ZnSO4 and FeSO4) were used with
consideration to the salt effect of the microemulsion (Table 3, entries
2-6). The yield of phenolic monomers is 69.2 mg g-1 in
the presence of FeSO4, but much higher yields are
provided by CuSO4 and ZnSO4 (89.3 and
88.9 mg g-1 respectively), while the highest yield ofp -hydroxy benzaldehyde (32.2 mg g-1) is
achieved in the presence of CuSO4 (Table 3, entries 4
and 5). These differences between metal sulfates can be attributed to
the different redox potential of metal cations.44However, there is negative effect for lignin oxidation in the acidic
condition under the catalysis of MgSO4, because it
belongs to alkali salt (Table 3, entry 6). Besides, the influence of
metal sulfates with monovalent and trivalent cations
(Na2SO4 and
Fe2(SO4)3) to lignin
oxidation was also investigated (Table 3, entries 7 and 8). But they
both show much lower catalytic activity than that of
CuSO4. It should be noticed that the yield of phenolic
monomers in the present of
Fe2(SO4)3 is lower than
that of FeSO4, which will be oxidized completely to
Fe2(SO4)3 at the
atmosphere of O2 (Table 3, entries 3 and 8). This
difference can be considered to trigger more serious peroxidation of
products under the catalysis of
Fe2(SO4)3.
Table 3. Comparison of metal sulfates on lignin oxidation in
the microemulsion.